1. Field of the Invention
This invention relates to chemical vapor deposition reactors for the growth of epitaxial layers and in particular to such reactors which reduce slip.
2. Description of Related Art
One of the problems associated with growing epitaxial films using prior art chemical vapor deposition (CVD) reactors is that wafers formed in such reactors often have defects known as slip. Slip is a dislocation of molecules along crystallographic planes formed when parts of a single crystal move relative to each other along these planes. This movement usually occurs when the wafer is unevenly heated or cooled during the deposition process, particularly between the top and bottom surfaces of the wafer and at the outer edges where heat is dissipated quicker. Thermal stress is then produced at portions of the wafer where large temperature gradients exist, causing slip when parts of the crystal move to relieve such stress. Wafers having slip defects can lead to undesirable yield loss when used to fabricate semiconductor devices.
In an attempt to eliminate slip, McNeilly et al. propose a method and apparatus in U.S. Pat. No. 4,496,609, entitled "Chemical Vapor Deposition Coating Process Employing Radiant Heat and a Susceptor", for using light sources placed outside the reactor to uniformly heat wafers mounted on a susceptor. Radiant energy from the light sources or lamps passes through the reactor walls, but is absorbed by the susceptor and/or wafers, thereby heating the wafers while keeping the walls cool. Because heat loss is greatest at the outer edges, the susceptor, which also serves as a heat sink, helps minimize temperature gradients at the outer edges, thereby increasing the temperature uniformity along the bottom of the wafers.
However, even with lamps carefully placed and controlled, it is difficult to maintain temperature uniformity on the wafers, especially along the tops of the wafers. As a result, slip defects may still occur on wafers subjected to epitaxial growth using the process and apparatus disclosed in McNeilly et al.
Accordingly, it is desired to have a structure and method which reduces slip during the manufacture of wafers in a CVD reactor.